1. Field of the Invention
This invention relates to a pneumatic tire having particularly an improved steering stability in the high-speed running of a vehicle by increasing a self-aligning torque of the tire.
2. Description of Related Art
In the running of the vehicle, the steering stability is largely affected by disturbances such as road surface state, wind and the like, which becomes serious as the vehicle speed is increased. In the conventional pneumatic tire, however, examinations on the shape of the block, formation of sipe and the like are not made for sufficiently satisfying the steering stability in the high-speed running of the vehicle.
It is, therefore, an object of the invention to provide pneumatic tires advantageously improving the steering stability in the high-speed running by directly increasing self-aligning torque of the tire through block, sipe and the like as a result of various studies with respect to the block shape and formation of sipe or fine groove therein.
According to a first aspect of the invention, there is the provision of in a pneumatic tire having a block pattern of a tread defined by a plurality of circumferential grooves extending in a circumferential direction of the tread and a plurality of lateral grooves each extending across the circumferential grooves and tread ends, the improvement wherein each block in an outermost block row in a widthwise direction is provided on its leading edge portion with a chamfered face gradually decreasing a height of the block toward a leading edge, and each block in the other block rows is provided on its trailing edge portion with a chamfered face gradually decreasing a height of the block toward a trailing edge, and a length of a chamfered face forming region is not more than ⅓ of a circumferential length of the block.
When a slip angle is applied to this pneumatic tire, the block in the outermost block row located opposite to the applying direction of the slip angle contacts with ground at a large area under a large loading and hence a force in a forward direction produced in the leading of the block is weakened by the chamfered face formed on the leading edge portion of the block to relatively create a large force backward to a forward rotating direction of the tire as compared with the block of the other block row provided on the trailing edge portion with the chamfered face, whereby restoring moment or self-aligning torque is caused in the ground contact region of the tread. As a result, even when the tire is subjected to disturbance or the like, it is possible to smoothly and rapidly return the tire in a running direction of the vehicle. Therefore, the excellent steering stability can be realized even in the high-speed running of the vehicle.
In a preferable embodiment of the first aspect, the chamfered face is formed in a region corresponding to not more than ⅓ of the circumferential length of the block, preferably not less than {fraction (1/10)} thereof, whereby the ground contact area of the block is sufficiently ensured.
When the length of the chamfered face forming region exceeds ⅓ of a circumferential length of the block, the ground contact area of the block lowers and hence cornering power important for improving the steering stability is lacking, while when it is less than {fraction (1/10)}, the effect by the formation of the chamfered face is actually poor.
In another preferable embodiment of the first aspect, a height of the chamfered face forming region is not more than ⅓ of a height of the block, preferably not less than {fraction (1/10)} thereof. In this case, a big self-aligning torque can be created to effectively ensure the improvement of the steering stability during the high-speed running. When the height exceeds ⅓, the ground contact area is decreased too much and hence the cornering power is decreased to cause a fear of degrading the steering stability, while when it is less than {fraction (1/10)}, the actual effect is lacking.
In the other preferable embodiment of the first aspect, the chamfered face is a curved face convexly extending outward in a radial direction of the tire. In this case, the ground contact pressure is uniformized to control the lowering of the cornering power as compared with the case of forming a flat chamfered face or the like.
According to a second aspect of the invention, there is the provision of in a pneumatic tire having a block pattern of a tread defined by a plurality of circumferential grooves extending in a circumferential direction of the tread and a plurality of lateral grooves each extending across the circumferential grooves and tread ends, the improvement wherein a plurality of sipes each extending in a widthwise direction of the tread and being symmetry with respect to a line segment passing through a center of the block in the circumferential direction of the tread are formed in each block of the block rows so as to gradually change lengths of these sipes from an end of the block in the circumferential direction toward the other end thereof, and the sipes in each block of an outermost block row become long at a leading side of the block and short at a trailing side thereof, and the sipes in each block of the other block rows become long at a trailing side of the block and short at a leading side thereof.
In this tire, each sipe formed in each block and extending in the widthwise direction of the tread can naturally develop an excellent water film cutting performance under its edge action but also particularly decreases a block rigidity at the leading edge portion in each block of the outermost block row. As a result, when a slip angle is applied to the tire, a force in a forward direction produced in the leading of the block is decreased to relatively increase a force in a backward direction, while a block rigidity at a trailing edge portion is decreased in each block of the other block rows to relatively increase a force in a forward direction. Even in this case, the self-aligning torque can effectively be increased based on force couple in the ground contact region of the tread, whereby the excellent steering stability can be realized during the high-speed running of the vehicle.
In a preferable embodiment of the second aspect, both ends of each sipe in each block are terminated in the block, whereby the excessive lowering of the block rigidity is prevented to control the occurrence of uneven wear.
According to a third aspect of the invention, there is the provision of in a pneumatic tire having a block pattern of a tread defined by a plurality of circumferential grooves extending in a circumferential direction of the tread and a plurality of lateral grooves each extending across the circumferential grooves and tread ends, the improvement wherein each block in all block rows is provided with a single fine groove extending in a widthwise direction of the tread across the block, and the fine groove formed in each block of an outermost block row is located in a leading edge portion of the block and the fine groove formed in each block of the other block rows is located in a trailing edge portion of the block.
Even in this tire, a block rigidity in each block of the outermost block row is lower at the leading edge portion than at the trailing edge portion, while a block rigidity in each block of the other block rows is lower at the trailing edge portion than at the leading edge portion, so that the self-aligning torque can be increased to bring about the improvement of the steering stability during the high-speed running likewise the previously mentioned tires.